Energy derived from expansion of liquefied gas



M. J. FRENCH Aug. 31, 1965 ENERGY DERIVED FROM EXPANSION OF LIQUEFIED GAS Filed July 20. 1961 y Inventor /f/'Cbae/ JUSQU/7 f/eC/? B 7/2 Mide# (VAGSSQW m out loog .E .mmmev me@ FS M- J. FRENCH Aug. 31, 1965 ENERGY DERIVED FROM EXPANSION 0F LIQUEFIED GAS Filed July 20. 1961 mmL OWN..

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OW, w 4%* A Horne y VUnited States Patent O 3,203,191 ENERGY DERIVED FROM EXPANSION F LIQUEFIED GAS Michael Joseph French, New Malden, England, assiguor to Couch International Methane Limited, Nassau, Bahamas, a company of the Bahamas Filed July 20, 1961, Ser. No. 125,398 Claims priority, application Great Britain, Sept. 2, 1960, 30,348/ 60 11 Claims. (Cl. 62-9) This invention relates to improvements in processes for the liquefaction of gases.

Processes for the liquefaction of gases include those in which the gas is liquefied under high pressure, usually by mechanical refrigeration, and the pressure on the liquefied gas is then reduced to produce a cold liquid at a lower pressure. Typical examples of such processes are described in U.S. Patent No. 2,896,414, and the copending application Ser. No. 698,667, filed November 25, 1957, now U.S. Patent No. 3,020,723, and the copending appli cation Ser. No. 114,199, filed lune l, 1961. Heretofore, the reduction in pressure on the liquefied gas has been achieved by passing the liqueed gas through an expansion valve. This method involves the conversion of a considerable proportion of the liquefied gas into a gas phase, and the gas phase so produced has to be recycled and reliquefied. It is clear that any method of reducing the amount of gas formed during this pressure reduction step will result in a saving in the horse power required to operate the liquefaction plant.

I have now found that, if the liquefied gas be passed through a hydraulic motor under such conditions that substantially no gas is produced in the motor before being passed through a conventional expansion valve, then the amount of vapour produced for a given pressure reduc- 'tion is far less than if the expansion valve were used on its own.

Accordingly, the present invention provides a process for the liquefaction of a gas in Which the gas is liquefied under a high pressure and the pressure of the liquefied gas is then reduced, the method of effecting such reduction in pressure comprising passing the liquefied gas at 4the high pressure through a hydraulic motor under such conditions that substantiallyno gas phase is formed in the motor and energy is produced.

The conditions necessary to ensure that no gas phase is formed in the hydraulic motor are achieved by maintaining the back pressure on the motor in excess of the saturation pressure of the liquid.

Generally, the liquefied gas Will require further cooling after passing through the hydraulic motor and and this may be achieved by then passing the liquefied gas through an expansion means in which part of the liquid is converted to gas and the pressure and temperature of the liquid are reduced to the required level. However, other means of further cooling may be employed in suitable circumstances, e.g. further refrigeration by heat exchange.

In this specification, the term hydraulic motor includes any motor driven by passage of a liquid through it. It thus includes hydraulic turbines of the impulse or Pelton type, of the reaction or Francis type or of the propeller or axial flow type, and also displacement machines.

This invention is applicable to the liquefaction of any gas, such as oxygen, nitrogen, helium or hydrogen, but is particularly useful in the liquefaction of hydrocarbon gases, such as natural gas or methane or ethane.

The following shows the savings that can be achieved by the present invention.

If liquid methane at a pressure of 3000 pounds per square inch and a temperature of 245 F. is passed through a hydraulic turbine to let the pressure down to Cce 50 pounds per square inch, and then through an expansion valve to reduce its pressure to 15 pounds per square inch and its temperature to 258 F., the amount of gaseous methane produced is about a third of the amount of gaseous methane produced when the liquefied methane at 3000 pounds per square inch and 245 F. is passed through an expansion valve only to reduce its pressure to 15 pounds per square inch and its temperature to 258 F. This means that there is a reduction of about two thirds in the energy required to reliquefy the gaseous methane. In addition, the energy produced by the hydraulic turbine is the equivalent of 20 B.t.u./ pound methane which is a further saving.

The process of the present invention is applicable to all liquefaction processes in which the gas is liquefied under high pressure, and the pressure of the liquefied gas is then reduced, irrespective of the refrigeration system employed. Thus, for example, a gas expansion process of refrigeration or a vapour compression or two-phase process of refrigeration may be employed.

The invention will now be illustrated by reference to the accompanying drawing which represents a ow sheet for the liquefaction of natural gas using a gas expansion process ot refrigeration.

Natural gas at a pressure of 465 p.s.i.a. and a temperature of F. is fed through pipe 1 into a two-stage reciprocating compressor 2 in which it is compressed to 3000 p.s.i.a. Cooling between the two stages of the compressor is achieved by water in inter-cooler 3, and the compressed natural gas passes from the compressor by pipe 4 through a further water cooler 5 and then through two heat exchangers 6 and 7 in parallel. In these heat exchangers, the temperature of the compressed natural gas is reduced to 247 F. and the gas is liquefied. The liquefied gas then passes through a Peiton wheel S from which it issues still as a liquid at a temperature of 224 F. and a pressure of 55 p.s.i.a. The power output of the Felton wheel which, for a flow of 240,000 lbs. per hour of natural gas, will amount to about 1500 horsepower, may be used in any convenient manner, for example in driving one or more of the compressors in the system.

The liquefied natural gas leaves the Pelton wheel through line 9 and passes through expansion valve 10 into the fiashing vessel 11. The pressure in the flashing vessel 11 is 16 p.s.i.a., and the temperature 264 F. The liquefied natural gas collecting in flashing vessel 11 is finally pumped to storage tank 12 via pump 13.

The gas phase at 264 F. formed in flashing vessel 11 is led by line 14 through heat exchanger 6 to condense some of the high pressure natural gas and is then recompressed to 50 p.s.i.a. in compressor 15 for use as fuel. Similarly, the boil-off from the storage tank 12 is led through line 16 and compressors 17 and 15 for subsequent use as fuel.

The refrigerating means in heat exchanger 7 which effects the refrigeration of the bulk of the pressurized natural gas is pipe 18 which carries gaseous nitrogen at 118 p.s.i.a. and 250 F. At the heat exchanger exit, the temperature of the nitrogen is 89 F. and its pressure 103 p.s.i.a. The nitrogen is then compressed in compressor 19 to 2312 p.s.i.a., cooled to 33 F. in heat exchanger 20 against liquid ammonia in line 22 and expanded back to 118 p.s.i.a. and a temperature of 250 F. in expander 21 to complete the refrigeration cycle. Expander 21 provides some o the power for driving the compressor 19. The ammonia refrigeration cycle for cooling in heat exchanger 20 may be of any conventional type and is not shown in the drawing.

ln the above-described process, other refrigerants may be used if desired, for example methane can be used in place of nitrogen, and propane in place of ammonia.

Another particularly useful application of the present invention is in connection with the invention described in the aforementioned copending application Serial No. 114,199 and can be applied, for example, by passing the liquefied natural gas leaving heat exchanger 4 through a hydraulic turbine before it passes through expansion valve 6 as shown in the drawing accompanying that application.

Another useful application of the present invention is in connection with the invention described in the aforementioned copending application Serial No. 698,667, now Patent No. 3,020,723. That invention provides a method of liquefying natural gas comprising inter alia the steps of supplying the gas in a process stream at an elevated temperature and pressure, removing heat from the process stream to reduce the natural gas to a temperature at which it is condensed to a liqueed state at the pressure conditions existing, and expanding the liqueed gas to lower pressure for storage and transportation. Heat is removed from the process stream by heat exchange with a plurality of separate refrigerants, each of said refrigerants passing though a compession and expansion cycle, the expansion step being subdivided into a higher pressure section and a lower pressure section, the refrigerant owing from the higher pressure section to the lower pressure section and the process stream passing in heat exchange relationship first with the higher pressure section and then with the lower pressure section.

The pressure reduction step in that method may be accomplished in accordance with the present invention. Thus, the liqueed natural gas stream leaving heat exchanger 144 may be passed through a Pelton wheel before it passes through throttling valve 146 as shown in FIG URE 1 of the drawings accompanying said copending application Serial No. 698,667. Similarly, the pressure reducing valve 156 in the said FIGURE 1 could be preceded by a Pelton wheel.

The expanion means through which the liquefied gas is passed after leaving the hydraulic motor may be an expansion valve or throttle valve or an expansion engine which may be of the reciprocating piston type or a turbine.

The work developed by the hydraulic motor and also Iby the expansion engine if used may of course be used for any suitable purpose such as driving the compressors required in the liquefaction cycle or for generating electricity.

In many liquefaction processes, the liqueed gas stream has to be subjected to pressure reduction in a number of stages, and the method of the present invention may be used in any one or more of these stages as may be found convenient.

I claim:

1. A process for the liquefaction of a gas in which the gas is liqueiied under a high pressure and the pressure of the liquelied gas is then reduced, the method of effecting such reduction in pressure comprising passing the liqueed gas at the high pressure through a hydraulic motor while maintaining the back pressure on the motor in excess of the saturation pressure of the liquid whereby substantially no gas phase is formed in the motor and energy is produced.

2. A process as claimed in claim 1 in which, after passing through the hydraulic motor, the liqueed gas is then passed through an expansion means in which part of the liquid is converted to gas and the pressure and temperature of the liquid are reduced to the required level.

3. A process as claimed in claim 2 in which the expansion means in which part of the liquid is converted to gas is an expansion valve.

4. A process as claimed in claim. 1 in which the hydraulic motor is a hydraulic turbine.

5. A process as claimed in claim 4 in which the hydraulic turbine is a Pelton Wheel.

6. A process as claimed in claim 1 in which the gas is liquefied under high pressure by heat exchange against a refrigerant operating in a gas expansion process of refrigeration.

7. A process as claimed in claim 1 in which the gas is liqueed under high pressure by heat exchange against a refrigerant operating in a vapour compression process of refrigeration.

8. A process as claimed in claim 1 applied to the liquefaction of natural gas.

9. A process for the liquefaction of a gas in which the gas is liqueed under high pressure and the liquefied gas is then passed at the high pressure through a hydraulic turbine while maintaining the back pressure on the motor in excess of the saturation pressure of the liquid whereby substantially no gas phase is formed in the turbine and energy is produced and then passed through an expansion valve in which part of the liquid is converted to gas and the pressure and temperature of the liquid are reduced to the required level.

10. A process as claimed in claim 9 in which the hydraulic turbine is a Pelton wheel.

11. A process as claimed in claim 9 applied to the liquefaction of natural gas.

References Cited by the Examiner UNITED STATES PATENTS 2,495,549 1/50 Roberts 62-39 2,552,451 5/51 Patterson 62--29 2,7 60,35 6 8/ 5 6 Sixsmith 62-9 2,896,414 7/59 Tung 62-40 2,901,326 8/59 Kurata 62-38 3,066,492 12/ 62 Grunberg 62-40 FOREIGN PATENTS 351,391 6/31 Great Britan. 864,855 4/ 61 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

ROBERT A. OLEARY, Examiner. 

1. A PROCESS FOR LIQUEFACTION OF A GAS IN WHICH THE GAS IS LIQUEFIED UNDER A HIGH PRESSURE AND THE PRESSURE OF THE LIQUEFIED GAS IS THEN REDUCED, THE METHOD OF EFFECTING SUCH REDUCTION IN PRESSURE COMPRISING PASSING THE LIQUEFIED GAS AT THE HIGH PRESSURE THROUGH A HYDRAULIC MOTOR WHILE MAINTAINING THE BACK PRESSURE ON THE MOTOR IN EXCESS OF THE SATURATION PRESSURE OF THE LIQUID WHEREBY SUBSTANTIALLY NO GAS PHASE IS FORMED IN THE MOTOR AND ENERGY IS PRODUCED. 